Microwave sensing apparatus and method for burner control



IN 010 ATO R SERVO DEVICE SIGNAL AMPLIFIER -28 F. J. WRIGHT Filed 001,. 1, 1965 30) ERROR AMPLIFIER INVENTOR. FRANKLIN J. WRIGHT BY 6 W Qwgyb ATTORNEY MICROWAVE SENSING APPARATUS AND METHOD FOR BURNER CONTROL Dec. 20, 1966 R 4 I D 3 4 .m m A 2 F 2 AM 4 M m m T C E T. E D P 9 .1 2 3 \r/ A L a H w 6- /l 4 s 2 r 4% a 8 P m m 2 A m 5 m I W E 1 T S T L L I w A W W 4 ll-IIIILJII 9 T\ I United States Patent 3 292,855 MICROWAVE SENSING APPARATUS AND METHOD FOR BURNER CONTROL Franklin J. Wright, Watchung, NJ., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed Oct. 1, 1965, Ser. No. 492,161 Claims. (Cl. 236-15) This invention relates to the control of burners. More particularly, the invention relates to the control of burners by adjustment of the fuel-air ratio in response to the characteristics of the burner flame. Specifically, the invention relates to the use of microwaves for determining the electrical conductivity of a burner flame and adjusting the fuel-air ratio to provide maximum combustion efliciency of a burner or a plurality of burners.

There exists a real need of means for monitoring the fuel-air ratios of burners in industrial furnaces, particularly when the burners are operated under conditions of low excess air. The problem is multiplied when a furnace has a plurality of burners supplied with fuel and air from manifolds because of the difficulty of maintaining the proper fuel-air ratio for each burner in the furnace.

The devices presently available all have serious shortcomings. It is for example, undesirable to use either probes or thermocouples because they are expensive and corrode easily. Optical devices often fail to function properly because of sooting and clouding of the windows.

The use of microwaves as disclosed in the present invention provides a reliable solution to the problem of determining fuel-air ratio with the desired degree of accuracy and speed. The apparatus and method of the invention will also detect the presence of absence of a flame.

The gases in a flame are partially ionized and the fuelair ratio is a function of the degree of ionization. This relationship can be plotted as shown by Calcote et al., Fifth Symposium (International) On Combustion, Reinhold, 1955 at page 433. Thus, for any particular fuel, the optimum fuel-air ratio can be determined. I have found that microwaves can be used to optimize burner operation and furnace operation.

The object of this invention is to provide a method and apparatus for continuous burner control and for maximizing the efliciency of a furnace by interrelated control of the burners.

Generally speaking, the invention involves the following steps: A beam of monochromatic microwave radiation from a microwave generator, after suitable attenuation, is made to pass through the flame. After further attenuation, the microwaves are received by a detector and passed on to an amplifying and measuring system. This information is used to adjust and control the quantity of air supplied to the individual burners or to groups of burners. The information can also be used to adjust and control the fuel supply as well as the air supply to the burners and/ or the fuel and air manifolds.

The invention will be more fully described with reference to the attached drawing which is a schematic view of the apparatus showing one arrangement of the apparatus elements.

Referring to the drawing, reference numeral 1 denotes a direct current power supply having a rating of 200 to 3000 volts. Connector 2 connects the power supply with low power klystron 3. The klystron generates monochromatic microwaves in the range of 30 to 300 megacycles per second.

The microwaves are passed by connector 4 to isolator 5 which serves to propagate the waves in the desired direction and by connector 6 to attenuator 7 which acts to decrease the magnitude of the signal. The waves are then passed by connector 8 to a switch S1 which directs them 3,292,855 Patented Dec. 20, 1966 via connectors 9, 10 or 11 to antennas 12, 13 or 14. The drawing shows the switch joining lines 8 and 10 so that the microwave signal is being passed to antenna 13. Dotted lines at switch S1 show how lines 8 and 9 can be joined by the switch to pass the signal to antenna 12 and how lines 8 and 11 can be joined to pass the signal to antenna 14. The electrical switch S1 is timed to direct the signal for a fraction of a second to each of antennas 12, 13 and 14 and then to repeat the cycle. Thus, one microwave generating system is capable of serving a plurality of burners.

Antennas 12, 13 and 14 orient the signal through the central portion of the flames 15, 16 and 17 of burners A, B and C. Another set of antennas designated 18, 19 and 20 receive the waves which pass through the respective flames. The antennas are attached to the interior furnace walls at suitable locations so that they are oriented with the particular flame being monitored. If desired, both antennas can be located on the same ide of the furnace and the microwave signal reflected back by means of a reflector. The walls of the furnace and other furnace structure have not been shown in the drawing. The wave signal is passed from the receiving antennas by connectors 21, 22 and 23 to a switch S2. The drawing shows switch S2 joining lines 22 and 24 so that the signal which has passed through flame 16 and receiving antenna 19 is passed to detector 25. Switch S2 is synchronized with switch S1. The dashed line 26 joining switches S1 and S2 represents a suitable synchronizing device which may be a mechanical linkage or an electrical unit.

The signal from detector 25 is passed by conductor 27 to amplifier 28 and then by conductor 29 to the error amplifier 30 where it is matched against the command received from attenuator 7 by onductor 31. The error amplifier serves to check variations in the microwaves generated by the generating system. The signal from the error amplifier is passed by conductor 32 to the servo device 33 which is also connected to the signal amplifier 28 by connector 34. The servo device is linked by connector 35 to a visual indicator 36 and thence by connector 37 to switch S3. Indicator 36 contains a differential measuring circuit for measuring the degree of deviation from the desired microwave characteristics of the flame being observed and a corrective signal is passed to the burner by line 37.

Since the fuel-air ratios of most burners are adjusted by adjusting only the air, switch S3 is connected by lines 38, 39, and 40 to air regulators 41, 42 and 43 which supply primary air to burners A, B and C, respectively. In the drawing, switch S3 is shown connecting line 37 to line 39 and air regulator 42 which serves burner B. Switch S3 is synchronized with switch S2 by a suitable synchronizing device represented by line 44. Thus, switches S1, S2 and S3 in the positions shown are synchronized to adjust the fuel-air ratio of burner B.

Air is supplied by line 45 to air manifold 46. Lines 47, 48 and 49 supply air to burners A, B and C, respectively. In this embodiment the manifold supplies three :burners; however, any desired number of burners can be supplied .by the manifold. The air regulators on the burners can be conventionally operated by electric motor valves or any other type of automatic valve.

Fuel is supplied by line 50 to fuel manifold 51 and thence to the individual burner-s by lines 52, 53 and 54. The flow of fuel is usually preset and maintained at a constant rate while adjustments are made by varying the air rate. If desired both the fuel and the air rate can be controlled for automatic adjustment in response to the indicated variations in microwave measurements.

Assuming for example, that burner B is operating with an excess quantity of air, the ion characteristics of flame 16 would be such that the microwave signal received by 3 antenna 19 and transmitted by switch S2 and the connectors and units designated 24, 25, 27, 28, 29, 30, 32, 33 and 35 to indicator 36 would be indicative of this condition. Line 37 would transmit an adjustment signal to air regulator 42 through switch S3 and line 39. In this manner burner B would be almost instantaneously adjusted to the proper fuel-air ratio.

Those skilled in the art can select the proper microwave components from commercially available units such as those described in the text Microwave Theory and Measurements, Staff-Hewlett-Packard Co., published by Prentice-Hall, 1963. Selection of components will depend on the type of furnace, burners and fuels employed as well as the characteristics of the specific flames to be monitored.

The process and apparatus of the invention provides many advantages. The microwave antennas do not have to be placed in or close to the burner flames and thus are not subject to heat damage, sooting, temperature variations, etc. invention will see this immediately. The combination of microwave detection of flame characteristics and electrical controls provides a system in which changes are observed and corrective action taken within seconds. Therefore, furnaces having a plurality of burners can be operated with the highest order of efficiency.

The invention can be applied to flames derived from the combustion of solid, liquid, vapor or gaseous fuels. The invention is particularly applicable to flames resulting from the combustion of industrial fuels such as natural gas, coal, furnace oils and gasoline. Most particularly, the invention is applicable to liquid fuel having an initial boiling point above about 300 F., e.g., residual fuels.

What is claimed is:

1. A microwave burner control system for a plurality of burners having a common fuel source and a common air source comprising means for determining the electrical conductivity of each burner flame, said means including means for propagating monochromatic microwaves transversely through the center region of each burner flame, means for detecting the propagated waves, means for indicating variations in fuel-air ratio as a function of variations in the propagated waves, air control means adapted to vary the air supply to each burner, and connector means adapted to transmit electrical signals between the propagating means, the detecting means, the indicating means, and the air control means.

2. Apparatus comprising, in combination, a plurality of burners and control means .for analyzing and optimizing the fuel-air ratio of each burner, said control means comprising:

( 1) A source of monochromatic microwaves,

Should a flame go out, the apparatus of the (2) First switch means to sequentially direct the micro-' waves to each of the burner flames in a desired order,

said microwaves passing transversely through the central portion of each respective flame, (3) Second switch means, synchronized with said first switch means, to sequentially direct microwaves 1 which pass through the flames to detecting and indicating means in the same order, (4) Individual means associated with each of said burners for adjusting the fuel-air ratio in response to variationsin the said microwaves,

(5) Third switch means, synchronized with said first and second switch means, to adjust said individual means in the same order employed to direct, detect and indicate the said microwaves.

3. A method of monitoring the fuel-air ratio of the flame of burners in an industrial furnace comprising the steps of directing a beam of attenuated monochromatic microwaves transversely through the central portion of the flame and measuring the loss of microwave energy.

4. A process for continuous microwave control of low excess air combustion which comprises directing a beam of substantially constant frequency microwaves transversely through the central portion of a flame, measuring the loss of microwave energy with a microwave detector and an indicator, employing the microwave signal information to. automatically control the air as it is fed to the burner and mixed for low excess air combustion.

5. A process for the control of combustion with low excess air ina multiple burner residual fuel-fired furnace provided with an air manifold and a fuel manifold which comprises adjusting the air flow to each burner so that the fuel-air ratio is an optimum value for complete com-.

busti'on of the residual fuel, said adjustment being made continuously and substantially instantaneously in response to the electrical conductivity of each burner flame as determined by a monochromatic microwave detection system wherein the microwaves are propagated transversely through the central region of the flame of each burner.

References Cited by the Examiner JAMES W. WESTHAVER, Primary Examiner. 

1. A MICROWAVE BURNER CONTROL SYSTEM FOR A PLURALITY OF BURNERS HAVING A COMMON FUEL SOURCE AND A COMMON AIR SOURCE COMPRISING MEANS FOR DETERMINING THE ELECTRICAL CONDUCTIVITY OF EACH BURNER FLAME, SAID MEANS INCLUDING MEANS FOR PROPAGATING MONOCHROMATIC MICROWAVES TRANSVERSELY THROUGH THE CENTER REGION OF EACH BURNER FLAME, MEANS FOR DETECTING THE PROPAGATED VAVES, MEANS FOR INDICATING VARIATIONS IN FUEL-AIR RATIO AS MEANS FOR VARIATIONS IN THE PROPAGATED WAVES, AIR CONTROL MEANS ADATPED TO VARY THE AIR SUPPLY TO EACH BURNER, AND CONNECTOR MEANS ADAPTED TO TRANSMIT ELECTRICAL SIGNALS BETWEEN THE PROPAGATING MEANS, THE DETECTING MEANS, THE INDICATING MEANS, AND THE AIR CONTROL MEANS. 